In the past, for the belt material of a pneumatic radial tire for a light truck (LT) or a recreational vehicle (RV), steel cords having twisted structures of 2+7, 2+6, 3+6, etc. were widely used. These twisted steel cords, however, are high in cost since two twisting processes are necessary for the twisting and are low in corrosion resistance since there is insufficient penetration of rubber into the steel cord, and therefore, there was the problem of the tire having a lower durability.
To eliminate this defect of the steel cords requiring two twisting processes, a flattened (or ellipse or oval) open structure of a 1.times.4 or 1.times.5 twist structure has been developed. However, as a result of studies by the present inventors, it has been found that a flattened open 1.times.4 or 1.times.5 twist structure is insufficient in strength as the belt material of a pneumatic radial tire for a light truck or a recreational vehicle. Therefore, consideration has been given to making it a 1.times.6 twist structure or a larger 1.times.7 twist structure. However, the inventors found that there was the defect that the cord shape became unstable in the case of a 1.times.7 structure.
However, even when adopting a flattened open 1.times.6 twist structure, it was necessary to make the flatness ratio (short diameter D.sub.S /Long diameter D.sub.L) smaller in order to ensure that the rubber fully penetrated into the steel cord. Therefore, the long diameter D.sub.L becomes inevitably larger. As a result, when arranging the cord so that its long diameter direction was along the planar direction of the belt layer, when the count of the steel cord is made equivalent to the count of the belt layer used in the past, the interval between cords becomes smaller, and therefore, the problem occurs that the durability of the belt edges falls.
Further, in the case of a 1.times.6 twist structure, there is the problem that the twist structure becomes unstable compared with the 1.times.4 or 1.times.5 twist structure and tends to cause defects in the twisting.